FIG. 1 schematically shows the anatomy of a normal human ear. The ear typically transmits sounds, such as speech sounds, through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. The cochlea 104 includes three chambers along its length, an upper chamber known as the scala vestibuli, a middle chamber known as the scala media, and a lower chamber known as the scala tympani. The cochlea 104 forms an upright spiraling cone with a center called the modiolus where the axons of the auditory nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to convert mechanical motion and energy and, in response, to generate electric pulses which are transmitted to the auditory nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the transducer function within the inner ear to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, there are several types of auditory prostheses that have been developed, such as cochlear implants, brainstem-implants, midbrain-implants or cortical implants, that electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along an implant electrode. The cochlear implant typically includes an electrode carrier having an electrode lead 109 and an electrode array 110, which is threaded into the cochlea 104. For brain-stem, midbrain and cortical implants, the electrode array is located in the auditory brainstem, midbrain or cortex, respectively. These electrodes may also be used for sensing neural tissue response signals, i.e., function as measurement electrodes.
FIG. 1 shows some components of a typical cochlear implant system where an external microphone provides an audio signal input to an external signal processor 111 which implements one of various known signal processing schemes. The processed signal is converted by the external signal processor 111 into a digital data format, such as a sequence of data frames, for transmission by an external coil 107 into a receiving stimulator processor 108. Besides extracting the audio information, the receiving stimulator processor 108 may perform additional signal processing, such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through the electrode lead 109 to the implanted electrode array 110. Typically, the electrode array 110 includes multiple stimulation contacts 112 on its surface that provide selective electrical stimulation of the cochlea 104.
For an audio prosthesis, such as a cochlear implant, to work correctly, some patient-specific operating parameters need to be determined in a fit adjustment procedure where the type and number of operating parameters are device dependent and stimulation strategy dependent. In some cases, the fitting is based on subjective responses from the patient, where the behavioural thresholds (T-level) and maximum comfort levels (C-levels) are determined. If the user is uncooperative or has extremely limited expressive language skills (e.g., very young children), the subjective responses are not sufficient for fitting. Objective physiological measures can assist in such cases. For example, the electrically evoked stapedial reflex (ESR), the electrically evoked auditory brainstem response (EABR), and the electrically evoked compound action potential (eCAP) are objective measures that may be used to assess the auditory nerve response to electrical stimulation.
To collect information about the electrode—nerve interface, a commonly used objective measurement is based on the measurement of Neural Action Potentials (NAPs) such as the electrically-evoked Compound Action Potential (eCAP), as described by Gantz et al., Intraoperative Measures of Electrically Evoked Auditory Nerve Compound Action Potentials, American Journal of Otology 15 (2):137-144 (1994), which is incorporated herein by reference in its entirety. The eCAP is a measure of electrically elicited synchronous VIII nerve (auditory nerve) activity, and eCAP measures are performed by stimulation with a single pulse, where the signal is recorded over time after stimulus. In this approach, the recording electrode is usually placed at the scala tympani of the inner ear. The overall response of the auditory nerve to an electrical stimulus is measured typically very close to the position of the nerve excitation. This neural response is caused by the super-position of single neural responses at the outside of the auditory nerve membranes.
These recordings show characteristic minima (N1, N2) and maxima (P1, P2, P3), where the difference in amplitude between P2−N1 (eCAP-amplitude) of the recorded signal is of special interest. FIG. 2 shows an example of measuring eCAP amplitude based solely on time since stimulation for a single response signal recording. The response signal is characterized by the amplitude between the minimum voltage (this peak is called typically N1) and the maximum voltage (peak is called typically P2), the so-called local extrema. These extrema among others represent the most prominent physiological landmarks of the ECAP signal. The amplitude of the eCAP at the measurement position is in most cases between approximately 10 μV and 1800 μV. One eCAP recording paradigm is the “amplitude growth function,” as described by Brown et al., Electrically Evoked Whole Nerve Action Potentials In Ineraid Cochlear Implant Users: Responses To Different Stimulating Electrode Configurations And Comparison To Psychophysical Responses, Journal of Speech and Hearing Research, vol. 39:453-467 (June 1996), which is incorporated herein by reference in its entirety. This function is the relation between the amplitude of the stimulation pulse and the peak-to-peak voltage of the eCAP.
The eCAP threshold is determined based on amplitude-growth functions or via expert systems using the correlation of the measured eCAP signal towards templates of typical noise and clear responses (see, e.g., U.S. Pat. No. 7,818,052 and International Appl. No. PCT/US2005/021207). Amplitude growth functions (AGF) are a sequence of measurements with increasing stimulation intensity, and since the eCAP-amplitude increases, the input (stimulation intensity)/output (eCAP amplitude) function (usually estimated by a linear function: out=in*eCAP slope+eCAP offset) can be used to extrapolate the maximal stimulation intensity where no eCAP signal is present, the eCAP threshold. Expert systems increase the stimulation intensity successively until a signal is present, and the minimal stimulation intensity necessary yields the eCAP threshold. eCAP based measures are affected by efferent and afferent nerve fibers, where only afferent fibers contribute to the auditory percept. To elicit an eCAP which can be recorded, the necessary minimal strength of the corresponding stimulus might be up to about 11 charge units (˜nano coulomb) at a rate ˜40 Hz (see, e.g., MAESTRO Software 4.1, ART-task, at 10 ms Measurement Gap). In a post-operative scenario, this might result for some individuals in a too loud perception.
One common method for fit adjustment in a cochlear implant is to behaviorally find the auditory threshold (THR) and maximum comfortable loudness (MCL) value for each separate electrode contact. For this, the stimulation charge on a selected electrode channel is usually increased in steps from zero until the THR or MCL level is reached in a subjective procedure (e.g., method of adjustments) or an objective procedure (e.g., eCAP or ESR). This increase can be either stimulation burst duration or stimulation burst amplitude or a combination thereof. For this procedure, constant amplitude stimulation bursts with about 10-1000 msec duration are usually utilized. See, e.g., Rätz, Fitting Guide for First Fitting with MAESTRO 2.0, MED-EL, Fürstenweg 77a, 6020 Innsbruck, 1.0 Edition, 2007. AW 5420 Rev. 1.0 (English_EU); incorporated herein by reference in its entirety. Typically, each electrode channel is fitted separately without using the information from already fitted electrode channels.
Behavioural thresholds and maximum comfort levels are determined using pulse trains. If the frequency or the current (for single pulses within the pulse train) is increased, the stimulation of the auditory nerve by the pulse train is louder for the cochlear implant user. The individual loudness (reported by the cochlear implant user) plotted against increasing frequency or current is called the behavioural loudness growth function (LGF). The linkage between subjective loudness perception and eCAP threshold is done using adjustment factors: linear dependency of THR with MCL using fixed values, eCAP threshold and eCAP slope.
Some patient-specific operating parameters for a cochlear implant include:
THR1 (lower detection threshold of stimulation amplitude) for Electrode Contact 1
MCL1 (most comfortable loudness) for Electrode Contact 1
Phase Duration for Electrode Contact 1
Amplitude for Electrode Contact 1
Pulse Rate for Electrode Contact 1
THR2 for Electrode Contact 2
MCL2 for Electrode Contact 2
Phase Duration for Electrode Contact 2
Amplitude for Electrode Contact 2
Pulse Rate for Electrode Contact 2
Compression
Parameters of frequency, e.g., electrode contact mapping
Parameters describing the electrical field distribution, e.g., spatial spread
Another common method for fit adjustment in a cochlear implant is to “frequency match” the implant in one ear with the other, contralateral ear in order to enable the auditory system to detect correctly Interaural Level Differences (ILD) and Interaural Time Differences (ITD) since the signal process of both ears is connected. Localization of sound is influenced by ILD and ITD cues, but in general pitch cues are regarded as the most important factors affecting the performance of cochlear implant users. Currently, the bilateral matching of pitch is done by matching pitch to electrodes solely based on subjective measures.